May/June Media Highlights: The Geological Society of America Bulletin

Boulder, Colo. – The May/June issue of the GEOLOGICAL SOCIETY OF AMERICA BULLETIN includes a number of potentially newsworthy items. Topics include: analysis of thrust faulting on Mars and implications for near-surface volatiles such as water and ice; a new look at the great Cascadia earthquake of 1700; and earthquakes and gigantic landslides in the Summer Lake basin (south-central Oregon). Two articles focus on the Dead Sea, describing its 4000-year lake level history and the impact of storms on its western escarpment.

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The paper presents a series of scaled sandbox experiments designed to represent caldera formation. The results show that the structures grew incrementally outward by a series of outward dipping faults followed by the development of a peripheral zone of extension and sagging. In contrast to previous experiments, a wide variety of complex structures were produced. The structures were dependant on the depth and orientation of the experimental chamber, and include, (1) trapdoor collapse along curved faults, (2) trapdoor collapse along linear faults, (3) piston collapse along polygonal faults, (4) incremental collapse along linear faults, and (5) complex collapse. Calderas in nature exhibit all of these styles and we suggest that magma chamber depth and orientation may have influenced their development.

This work presents the lake-level record of the Dead Sea during the past four thousand years. The Dead Sea is a terminal lake located at the Israel-Jordan border, and its level fluctuations reflect the past paleoclimate variations in its drainage basin and probably in the entire Levant.

The level fluctuations are described from outcrops exposed along the Dead Sea shores, which are comprised of different sediments deposited in various water depths. The past Dead Sea shores are visible in the outcrops as pebble ridges, wave-sorted sands, and evaporitic crusts that indicate in the modern Dead Sea the exact elevation of its level. The level fluctuations are dated by radiocarbon ages of organic remains collected from the sediments.

The results show that the Dead Sea level fluctuated within the range of 390-415 m below sea level (mbsl) during historic times. High stands occurred in the first and second century B.C. and the fourth century A.D., during the Roman and Byzantine periods, in the eleventh through twelfth century A.D., during the crusader period, and at the end of the nineteenth century A.D. The rises mark a significant change in the annual rainfall in the region, which likely exceeded the instrumentally measured modern average.

The curve also indicates drastic drops that exposed the sediments to erosion. The oldest and probably deepest drop in the lake level culminated during the fifteenth to fourteenth century B.C. after a retreat from a higher lake stand. The longest low stand occurred after the Byzantine period and continued until the ninth century A.D. This arid period coincides with the invasion of Moslem-Arab tribes into the area during the seventh century A.D. These level drops represent extreme arid conditions that occurred over the past several thousand years.

Variations in lithology and pore volatile pressure influence the distribution of layer and interface strength (mechanical stratigraphy) within the crust. We show how mechanical stratigraphy can be inferred from fault dip directions and relative fault lengths, which are derived from the topography of thrust fault-related folds. Applying this method on Mars, we find that thrust fault-related folds that have shorter secondary backthrust faults are spatially correlated with a general lithologic sequence of lava flows overlying older impact ejecta and young lobate ejecta craters on the lava flow surface — evidence of near surface volatiles, such as water ice. We demonstrate that secondary backthrusts within fault-related folds in the western equatorial region of Mars formed due to volatile-enhanced mechanical stratification of lava flow-ejecta lithologic sequences.

Geomorphic response to seasonal variations in rainfall in the Southwest United States
Devin Etheredge, Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA, et al. Pages 606-618.

Etheredge et al. compare streamflow and channel geometry characteristics in watersheds from two mountain ranges in Arizona. The geologic settings in the two ranges are chosen to be very similar in every respect except the seasonal cycle of precipitation: The northwest Arizona locale receives most of its precipitation in the winter from frontal systems (hence, widespread long-lived storms), whereas the southeast Arizona locale is monsoonal, receiving precipitation mostly from short but sharp summer thunderstorms. This climatic difference in precipitation delivery is clearly expressed in the geomorphic comparison of the two locales. The seasonal character of stream discharge in the monsoonal locale exhibits a pronounced dependence on basin size: Small basins respond quickly to summer storms, but larger basins yield largest discharges in response to winter precipitation. The dependence of stream flow seasonality on basin size is attributed to the small spatial scale of summer rainfall events.

Studies of mountain-building events along the former continental margin of East Antarctica ~500 million years ago provide insight into potential tectonic effects of subducting oceanic sea floor features, such as submarine ocean plateaus, on continental tectonics. The application of high precision dating techniques with geological rock relationships and regional deformation characteristics constrains the tectonic history of the Pensacola Mountains sector of the East Antarctic margin to have been dominated by geologically rapid switches between crustal shortening and extension. It is proposed that this style of continental margin deformation is likely to be the product of the subduction of thermally buoyant ocean floor features beneath the East Antarctic continent. Such models may explain the complex tectonic histories along convergent continental margins such as the Andes, where ocean floor is consumed beneath continental crust.

The last earthquake on the Cascadia subduction zone megathrust occurred in 1700, as evidenced by subsided marshes along the coastal margin and by Japanese tsunami records indicating a magnitude 9 event. Coastal subsidence in the 1700 event, as estimated from changes in marsh sediment, is compared with the subsidence predicted by computer models of elastic strain buildup and release. These models, on which seismic hazard assessments are largely based, are constrained by high precision geodetic data. Models that simulate the release of strain built up over ~550-800 years of plate convergence best agree with the marsh data. A further model/marsh data comparison shows that slip on the fault was consistent with a magnitude 9 earthquake.

Within the last several hundred thousand years, gigantic landslides have occurred along Winter Ridge, which bounds Summer Lake basin on the west. These landslides are typically a mile in width and more than a thousand feet thick. The landslides failed catastrophically, very rapidly running out more than a mile onto the basin floor. Recent engineering study has found that large earthquakes triggered these gigantic landslides. The source of the earthquakes is the active Slide Mountain-Winter Ridge fault that runs along the base of Winter Ridge. The fault is capable of producing magnitude 7 earthquakes. The last documented ground-rupturing earthquake on the fault occurred between 2,000 and 10,000 years ago.

The Northern Dabie Complex (NDC), which lies north of the famous central Dabie ultrahigh-pressure terrane in East Central China, has a controversial history. Detailed studies of gneisses and granites show that they have compositions and histories similar to rocks in the ultrahigh-pressure terrane and therefore that they are also part of the Yangtze craton and not of the Sino-Korean craton to the north. However, although the NDC was present at the time the two cratons collided in the Triassic, and it lies very close or adjacent to the suture, the studied rocks show no evidence of ultrahigh-pressure metamorphism such as found in the central Dabie.

Modern extreme storms and the rainfall thresholds for initiating debris flows on the hyperarid western escarpment of the Dead Sea, Israel Hagit
Ben David-Novak, Institute of Earth Sciences, Hebrew University of Jerusalem, Jerusalem, Israel, et al. Pages 718-728.

Debris flows deliver very large quantities of sediment in all mountainous environments of the world and cause heavy damage to life and infrastructure. Because of sparse observation, arid regions lack information on the rainfall that can cause such an extreme event; therefore, real-time hazard predictions are limited. In this research, we documented for the first time intensities and durations of rainfall that caused debris flows in the arid environment of the Dead Sea near the Ein Gedi Oasis, just north of the Massada historical site. Luckily, the storm occurred just over a dense net of rain gauges also covered by meteorological radar recording at relatively high temporal scale. Rarely occurring rainfall intensities exceeding 30 mm/hr for a duration of one hour (i.e., in 1 hour, over 60% of the mean annual rainfall in the area, which is ~50 mm) were required to initiate debris flows in this area. Therefore, we also concluded that the up to three of the debris flows that occurred in many of the very small basins that drain the Dead Sea escarpment during the last 3000 years were the results of extreme, very rare (low frequency) rain storms.

Glacial Lake Agassiz: A 5000 yr history of change and its relationship to the d18O record of Greenland
James T. Teller, Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada, and David W. Leverington, Center for Earth and Planetary Studies, National Air and Space Museum, Smithsonian Institution, Washington, D.C. 20560-0315, USA. Pages 729-742.

As glaciers retreated from North America at the end of the last ice age, Lake Agassiz formed and expanded northward along the margin of this drainage barrier. This giant lake was the largest body of water on the continent, extending over a total area of 1.5 million km² before catastrophically draining into the ocean about 8400 years ago. During its 5000-year history, the size and volume of Lake Agassiz changed abruptly on many occasions, periodically releasing floods of water into the oceans as glaciers melted and uncovered lower spillways from the lake. Some of the largest outbursts from Lake Agassiz occurred near the beginning of episodes of abrupt climate cooling that punctuated global warming at the end of the last ice age. These cool events are recorded in the isotopic record of the Greenland ice sheet and in the geological record over much of the Northern Hemisphere.

The Late Devonian Antrim Shale in the Michigan Basin is an economically significant source of microbially produced methane, along the basin margins. Stable isotope chemistry of shale formation waters shows that freshwaters, recharged from Pleistocene continental ice sheets and modern precipitation, suppressed basinal brine salinity to great depths and enhanced methanogenesis. Cl-Br-Na relations reveal that salinity is controlled not only by mixing between variable amounts of basinal brine and freshwater, but also by halite (NaCl) dissolution where fluids recharged through underlying Devonian carbonate aquifers with localized evaporite deposits. Ca, Mg, HCO3, and carbon isotopes of dissolved inorganic carbon have been systematically and profoundly altered by microbial methanogenesis. Large decreases in Ca/Mg and Ca/Sr ratios accompany increasing carbonate alkalinity values in areas with high rates of microbial gas production. These changes are consistent with calcite precipitation during progressive methanogenesis. Similar variations in fluid chemistry are evident in other sedimentary basins containing black shales and coal bed deposits associated with microbial gas.

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